Speaker adaptation of vocabulary for speech recognition

ABSTRACT

A phonetic vocabulary for a speech recognition system is adapted to a particular speaker&#39;s pronunciation. A speaker can be attributed specific pronunciation styles, which can be identified from specific pronunciation examples. Consequently, a phonetic vocabulary can be reduced in size, which can improve recognition accuracy and recognition speed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 10/320,020 filed Dec. 16, 2002, the complete disclosure of which, in its entirety, is herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to speaker adaptation of a vocabulary for speech recognition.

BACKGROUND

Speech recognition systems use a “phonetic vocabulary” that contains pronunciations of all the words that may be spoken by a speaker. Spoken words are matched against their recognized equivalent, and then provided to the speaker. Two performance criteria for speech recognition systems are speed and accuracy of recognition. Various refinements have been devised to improve these two performance criteria. The performance of a general purpose speech recognition system can be improved by adapting the system to a particular speaker. Many such refinements can be classified as either of two general types of adaptation mechanism.

The first kind of adaptation mechanism involves adapting acoustic models of speech used in the speech recognition system, and the second kind of adaptation mechanism involves adapting the vocabulary used by the speech recognition system.

Acoustic model adaptation (see Chin-Hui Lee, Chih-Heng Lin, Biing-Hwang Juang, “A Study on the speaker adaptation of the parameters of continuous density Hidden Markov Models,” IEEE Transaction on Signal Processing, Vol. 39, No. 4, April 1991) is generally used to improve recognition accuracy for a particular speaker, or in a particular environment. Acoustic model adaptation may be used in, for example, noisy environments, telephony environments, and office environments.

Vocabulary adaptation, by contrast, may be used in the context of performing particular task speakers (see A. Sankar, A. Kannan, B. Shahshahani, E. Jackson, “Task-specific Adaptation of Speech Recognition Models,” Proceedings of Automatic Speech Recognition and Understanding, ASRU, 2001). Particular vocabularies that are likely to be used are commonly context-dependent. A specific vocabulary is implied when the speaker is, for example, dictating technical correspondence, or performing certain command and control tasks.

There have been approaches wherein an adaptation of the vocabulary is achieved through changing the pronunciation networks (Kyung-Tak Lee, Lynette Melnar, Jim Talley, “Symbolic Speaker Adaptation for Pronunciation Modeling,” in ISCA Tutorial and Research Workshop on Pronunciation Modeling and Lexicon Adaptation for Spoken Language, Estes Park, Colo. USA, Sep. 14-15, 2002). This uses a pronunciation network to “generate” all the pronunciations of the words. Such a technique cannot work on choosing from the existing pronunciations that could have been created manually earlier.

These three types of adaptation mechanisms are responsible for improvements in the three above-mentioned performance criteria. Further advances in these performance criteria are welcome, and thus a need clearly exists for improved speech recognition techniques.

SUMMARY

A phonetic vocabulary for a speech recognition system can be adapted to a particular speaker's pronunciation. A speaker can be attributed specific pronunciation styles, which can be identified from specific pronunciation examples. Consequently, a recognized phonetic vocabulary can be reduced in size, which can improve recognition accuracy and recognition speed.

DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart representing steps involved in a technique described herein for improving speech recognition.

FIG. 2 is a flowchart representing steps involved in the technique of FIG. 1.

FIG. 3 is a block diagram representation of the technique.

FIG. 4 is a schematic representation of a computer system suitable for performing the techniques described with reference to FIGS. 1, 2 and 3.

DETAILED DESCRIPTION

A central difficulty involved in designing speech recognition systems is that different people pronounce words differently. Such alternative pronunciations must be taken into account by speech recognition systems. Alternate pronunciations of words are, in existing speech recognition systems, stored in a phonetic vocabulary to perform speech recognition. Table 1 below provides some examples in the English language. Table 2 below provides further examples in the Hindi language. The examples presented in Table 1 and 2 represent the pronunciations in terms of phones, the same principles apply to examples presented in other units of speech, such as dipones, or syllables.

TABLE 1 CAPITALIZE /K AE P AX T AX L AY Z/ /K AE P IX DX AX L AY Z/ ALIENATED /EY L IY AX N EY DX IX DD/ /EY L IY AX N EY T AX DD/ /EY L IY AX N EY DX IX DD/ END TABLE 1

TABLE 2 phl /PH AX L/ /F AX L/ zindgi /JH IH N DH AX G IY/ /Z IH N DH AX G IY/ bazi /B AA Z/ /B AA JH/ END TABLE 2

Maintaining all likely alternate pronunciations in a speech recognition system increases size of the vocabulary manifolds. Consequently, decreased recognition speed and lower accuracy of speech recognition can result.

Overview of Vocabulary Adaptation

The vocabulary used by a speech recognition system is adapted for particular speakers. This adaptation involves removing from the recognized vocabulary alternative pronunciations that are not used by a speaker. Those pronunciations that are used by the speaker are retained in the recognized vocabulary. Training can be performed to effect this adaptation, and may be performed during speaker enrollment, or an equivalent procedure.

FIG. 1 flowcharts steps involved in improving a speech recognition system using the approach described above. At enrollment, the speaker can be prompted to read a predetermined text to adapt existing acoustic models to the speaker. During this process, the identity of the speaker's speech is “aligned” with the predetermined text by matching corresponding phonetic units in the spoken speech and predetermined text.

During this alignment process, if a sentence contains some words that have more than one possible pronunciation, then the speaker's speech is aligned to all applicable alternate pronunciations of these words. For such words, the pronunciation that is best aligned with the speech is retained, and alternate pronunciations are discarded from the recognized vocabulary for that speaker. The speech alignment procedure can, for example, score each speech-to-word pronunciation that is represented in the speech. The most likely pronunciation can be identified by a high score rating. This alignment can be performed using existing techniques such as Dynamic Time Warping, Viterbi algorithm, Hidden Markov Models or some other available procedure.

A vocabulary containing all alternate pronunciations is initially retained. This initial vocabulary can further be used to create more speaker-specific vocabularies. This is required as a single installation of a speech recognition system can have more than one “user profile”. This technique can also be applied to progressively adapt the recognized vocabulary, as the speaker uses the speech recognition system.

Mechanics of Vocabulary Adaptation

The described vocabulary adaptation system operates by understanding a particular speaker's pronunciation style. For example, the words outlined in Table 3 below presents two alternate pronunciations of the sound corresponding to “ted” in their spellings.

TABLE 3 related, alerted, allocated, “ted” DX IX DD or T IX DD amelgameted, orchestrated concentrate, concept, concert, “on” AA N or AO N concord, conduit, upon aptitude, constitute, destitute, entity, “tit” T AX T or T IX T identity, institute, quantities, rectitude END TABLE 3

At speaker enrollment, the recognized vocabulary is adapted by putting some such words in the enrollment scripts. As the speaker speaks these words, an alignment scheme categorizes the speaker's style of pronunciation through certain “keywords”. The pronunciation of these keywords is used to adapt the alternate pronunciation of all the words in the domain of keywords in the vocabulary. Accordingly, the whole vocabulary can be adapted to a speaker's style of speaking without the speaker speaking all the words in a vocabulary.

Procedure

FIG. 2 flowcharts further steps involved in the described technique. Building a proposed adaptation system of the speaker vocabulary involves the following steps:

-   -   Step 210 Find prominent patterns that represent the variations         between alternate pronunciations of each word. These patterns         define the differences in the few phones that appear in the set         of alternate baseforms for a particular word. For example, if         the pronunciations for the word related are /R IH L EY DX IX DD/         and /R IH L EY T IX DD/, then the pattern that distinguishes one         baseform from the other is DX         T, meaning that DX can be replaced with T (or vice versa) to         form an alternate pronunciation for related. The objective is to         extract all such relevant patterns from the speaker-independent         vocabulary. These patterns serve as a distinguishing factor         between a pair of baseforms for the same word.     -   Step 220 Words that have similar phonetic alternations in their         alternate baseforms are bunched together. For example, words in         each row of the first column of Table 1 above are grouped as         one. Based on the speaker-specific pronunciation of one word, a         generalization can be made of the speaker's style of         pronunciation for all the words in the relevant category. So, if         a speaker speaks “related” as R IH L EY DX IX DD, then that         speaker's ambiguity for all words in that category is resolved         upon this basis, and other pronunciations of these words can         consequently also be removed from the vocabulary.     -   Step 230 Once such groups are formed, one word from each word is         chosen to be the “keywords” for that category.     -   Step 240 An enrollment script is formed that has all these         “keywords” in the script.     -   Step 250 Enrollment is performed and, as described earlier, the         best lexeme (alternate pronunciation) for the “keywords” are         selected by one of the schemes mentioned.     -   Step 260 Pronunciation rules for the speaker are learnt using         the speaker-specific pronunciations of the keywords.     -   Step 270 For each word in a specific category, only those         alternate baseforms of words are retained that follow the         user-specific rules and other alternate pronunciations are         discarded from the vocabulary.

Following the above steps for all the categories, the complete vocabulary is adapted to the user that has only those pronunciations that suits the speaker's style of speaking. All other pronunciations that are redundant to the user are deleted. This results in significant reduction in vocabulary size.

FIG. 3 is a block diagram that shows a system for vocabulary adaptation. The described technique for generating a user-adaptive vocabulary that has pronunciations selected in accordance with a speaker's pronunciation style. The various operational blocks are described below.

Categorization—310

This process groups all the words that have similar alternations in their alternate baseforms. This is a one-time process for a vocabulary and has to be performed on the SI vocabulary. Only those alternations that occur sufficiently often considered in the SI vocabulary. This process generates categories, each of which has words that have similar alternations in their pronunciations for a particular sound. For example, the words “alienated”, “regulated” and “educated” would all be in one category as all these words have two alternate pronunciations DX IX DD and T AX DD for the sound “ted” in all of them.

Pattern Generation—320

Using an algorithm similar to string matching with k-differences algorithm (referred to in T. Akutsu, “Approximate string matching with don't care characters,” Information Processing Letters 55, 235-239, 1995) for every word in the speaker-independent (SI) vocabulary, all the alternate pronunciations are used to generate the pattern 350 that represents the variations from one baseform to another. If a word w_(i) has N alternate baseforms {b₁, b₂, . . . , b_(N)}, each of the c₂ ^(N) possible pairs of alternate baseforms is considered to generate the patterns. This is a one-time process for a vocabulary and must be performed on the SI vocabulary.

Keyword Extraction—330

Once the words are grouped in categories, one or more keyword(s) are chosen for each category. These keywords are used in the enrollment script and are spoken by the speaker at the time of enrollment. The pronunciation of these keywords at the time of enrollment is used to generalize the assumed pronunciations of all the words in this category. For example, if, at the time of enrollment, a user speaks “regulated” as R EY G UH L EY T AA DD, then all the words in that category are assumed to be pronounced T AX DD for the sound “ted”. Alternate pronunciations are thus deleted. Therefore, keywords are representative of a class, and are used to decide on the pronunciations of all the other words in that category. This is also a one time process in building a recognition system.

Enrollment Generation Script—340

Since vocabulary adaptation is performed with speaker enrollment, the enrollment scripts 360 are created such that the relevant keywords are part of the scripts. Thus, the scripts 360 that are used for enrollment should include these relevant keywords in the scripts—360. As with the above two categorization and keyword processes, this is also a one-time process. The above four processes complete the backend requirements of a vocabulary adaptation system.

Alignment Generation—370

During speaker enrollment, those pronunciations of all the keywords that get best scores at the time of adaptation are stored along with the keywords. This step is performed at the time of speaker enrollment and this gives us the speaker specific pronunciations (out of the list of speaker independent ones) of each keyword.

Adaptation Rules Generation—380

Once pronunciations of each keyword are decided by the speaker adaptation system, rules are formed based on these pronunciations. Continuing the above example, the rule for the category of words (regulated, educated and alienated) would be that this speaker's vocabulary would have the sound “ted” in these words pronounced as “T AX DD”. The other pronunciation that contains /DX IX DD/ is thus deleted to form the speaker-adapted (SA) vocabulary.

Vocabulary Adaptation—390

Once all the rules are generated, the ambiguity in alternate pronunciations is removed by using these rules and a new speaker-specific vocabulary is generated. So the vocabulary now generated after speaker enrollment would have only one baseforms for the words (regulated, educated and alienated). This completes generation of speaker-specific vocabulary through one complete run of a vocabulary adaptation system.

A word can appear in multiple categories owing to the different sounds the word may have. For example, “reduced” is in one category owing to the multiple pronunciations of the sound “red” (R AX D or R IX D) and in another category owing to the sound “uced” (UW S TD or Y UW S TD). There can be more than one keyword for each category that can be put in the enrollment scripts. This provides greater confidence about the speaker's pronunciation of speaking that sound in a particular manner. Based on the speaker frequency of using the different alternate pronunciations, rules can be generated that take this into account and generate a SA vocabulary. For example, if there are N keywords representing the sound “ted” and the user pronounces some keywords with the sound /T IX DD/ and some with /DX IX DD/, having both the alternate pronunciations in the SA vocabulary for words in that category may be desirable.

Computer Hardware and Software

FIG. 4 is a schematic representation of a computer system 400 that can be used to implement the techniques described herein. Computer software executes under a suitable operating system installed on the computer system 400 to assist in performing the described techniques. This computer software is programmed using any suitable computer programming language, and may be considered as comprising various software code means for achieving particular steps.

The components of the computer system 400 include a computer 420, a keyboard 410 and mouse 415, and a video display 490. The computer 420 includes a processor 440, a memory 450, input/output (I/O) interfaces 460, 465, a video interface 445, and a storage device 455.

The processor 440 is a central processing unit (CPU) that executes the operating system and the computer software operating under the operating system. The memory 450 typically includes random access memory (RAM) and read-only memory (ROM), and is used under direction of the processor 440.

The video interface 445 is connected to video display 490 and provides video signals for display on the video display 490 for the benefit of the user. User input to operate the computer 420 is provided from the keyboard 410 and mouse 415. The storage device 455 can include a disk drive or any other suitable storage medium.

Each of the components of the computer 420 is connected to an internal bus 430 that includes data, address, and control buses, to allow components of the computer 420 to communicate with each other via the bus 430.

The computer system 400 can be connected to one or more other similar computers via an input/output (I/O) interface 465 using a communication channel 485 to a network, represented in this case as the Internet 480.

The computer software may be recorded on a portable storage medium, in which case, the computer software program is accessed by the computer system 400 from the storage device 455. Alternatively, the computer software can be accessed directly from the Internet 480 by the computer 420. In either case, a user can interact with the computer system 400 using the keyboard 410 and mouse 415 to operate the programmed computer software executing on the computer 420.

Other configurations or types of computer systems can be equally well used to implement the techniques herein, as is understood by those skilled in the relevant art. The computer system 400 is described only as an example of a particular type of system suitable for implementing the described techniques.

CONCLUSION

A method, a computer system and computer software are described herein in the context of improved speech recognition. Various alterations and modifications can be made to the techniques and arrangements described herein, as would be apparent to one skilled in the relevant art. 

1. A computer system for speaker adaptation of vocabulary for a speech recognition system, the computer system comprising: means for attributing a pronunciation style to the speaker, based upon the identified pronunciation; and means for restricting a vocabulary used in the speech recognition system to pronunciations consistent with the attributed pronunciation style.
 2. Computer software, recorded on a medium, for vocabulary adaptation for a speech recognition system, the computer software comprising: software code means for attributing a pronunciation style to the speaker, based upon the identified pronunciation; and software code means for restricting a vocabulary used in the speech recognition system to pronunciations consistent with the attributed pronunciation style.
 3. The computer system of as claimed in claim 1, further comprising a component operable for recording an enrollment text, including predetermined keywords, read by the speaker.
 4. The computer system as claimed in claim 3, wherein the keywords are representative of words having a phonetically similar baseform.
 5. The computer system as claimed in claim 3, wherein the keywords are representative of words having alternative pronunciations that are indicative of a likely pronunciation of similarly categorized words of the speaker.
 6. The computer system as claimed in claim 5, wherein the step of attributing a pronunciation style is based upon the pronunciation of the keywords of the speaker recorded in the enrollment text.
 7. The computer system as claimed in claim 1, wherein the step of attributing a pronunciation style is based upon the pronunciation of predetermined keywords of the speaker. 